Brian, Bruno,
Many thanks for your comments.
Based on the feedback I would propose to add Brian's proposed text as is into
the draft.
4.16.1. PSP: Penultimate Segment Pop of the SRH
SR Segment Endpoint Nodes advertise the SIDs instantiated on them via
control plane protocols as described in Section 8. Different
behavior ids are allocated for flavored and unflavored SIDs [Table 4]
such that an SR Source Node can identify PSP-flavored SIDs as such.
The PSP flavor is specifically used by the Source SR Node when it
needs to instruct the penultimate SR Segment Endpoint Node listed in
the SRH to remove the SRH from the IPv6 header.
SR Segment Endpoint Nodes receive the IPv6 packet with the
Destination Address field of the IPv6 Header equal to its SID
address. A penultimate SR Segment Endpoint Node is one that, as part
of the SID processing, copies the last SID from the SRH into the IPv6
Destination Address and decrements Segments Left value from one to
zero.
The PSP operation only takes place at a penultimate SR Segment
Endpoint Node and does not happen at any Transit Node.
The SRH processing of the End, End.X and End.T behaviors are
modified: after the instruction "S14. Update IPv6 DA with Segment
List[Segments Left]" is executed, the following instructions must be
executed as well:
S14.1. If (Segments Left == 0) {
S14.2. Update the Next Header field in the preceding header to the
Next Header value of the SRH
S14.3. Decrease the IPv6 header Payload Length by the Hdr Ext Len
value of the SRH
S14.4. Remove the SRH from the IPv6 extension header chain
S14.5. }
The usage of PSP does not increase the MTU of the IPv6 packet and
hence does not have any impact on the PMTU discovery mechanism.
As a reminder, [I-D.ietf-6man-segment-routing-header] defines in
section 5 the SR Deployment Model within the SR Domain [RFC8402].
Within this framework, the Authentication Header (AH) is not used to
secure the SRH as described in Section 7.5 of
[I-D.ietf-6man-segment-routing-header].
<NEW>
This behavior does not contravene section 4 of [RFC8200]
because the current destination address of the incoming packet
is the address of the node executing the PSP behavior.
</NEW>
Additionally, please find some replies inline to the comments from Bruno [PC2].
Thanks,
Pablo.
-----Original Message-----
From: "bruno.decra...@orange.com" <bruno.decra...@orange.com>
Date: Tuesday, 3 March 2020 at 15:49
To: "Pablo Camarillo (pcamaril)" <pcama...@cisco.com>, Brian E Carpenter
<brian.e.carpen...@gmail.com>
Cc: "Darren Dukes (ddukes)" <ddukes=40cisco....@dmarc.ietf.org>, 6man WG
<i...@ietf.org>, SPRING WG List <spring@ietf.org>
Subject: RE: PSP and a logical application of RFC8200
Pablo,
My reading is that Brian is asking for a clarification of the text, not a
change in the behavior.
In general, I think that clarification is good and that Brian's request is
reasonable.
Related to Brian's comment, but on top of them, I have further points on
this section 4.16.1:
1)
" S14.1. If (Segments Left == 0) {"
Although I agree that reading the whole pseudo code (split across the whole
document) is self-descriptive, there is the opportunity for people to misread
and confuse "Segment Left in the received packet" with "Updated Segment Left".
I'd like to avoid misunderstanding similar to the ones in RFC 8200. Could you
propose something? E.g. :s/ Segments Left/ Updated Segments Left.
PC2: In previous versions of this document we used the term “updated SL”. As
part of the WGLC we changed this text in December to “Segments Left” since it
is the name of the field defined in the SRH. While I do find the “Updated
Segments Left” easier to understand, the working group feedback wasn’t the same
in December. Hence I would tend to keep it as is. But no strong preference on
my side really.
2)
As a possible change to try to address Brian's comment
OLD:
The PSP operation only takes place at a penultimate SR Segment
Endpoint Node and does not happen at any Transit Node.
NEW:
The PSP operation only takes place at a penultimate SR Segment
Endpoint Node (where Segment Left == 1) and does not happen at any
Transit Node.
PC2: Given the new text added, and given that we already have this, do we need
it?
A penultimate SR Segment Endpoint Node is one that, as part
of the SID processing, copies the last SID from the SRH into the IPv6
Destination Address and decrements Segments Left value from one to
zero.
More inline
> From: ipv6 [mailto:ipv6-boun...@ietf.org] On Behalf Of Brian E Carpenter
>
> On 03-Mar-20 09:02, Pablo Camarillo (pcamaril) wrote:
> > Brian,
> >
> > The PSP pseudocode is presented as a modification to the End pseudocode
starting at line S14 of such.
> > Please go through the PSP pseudocode in conjunction with the End
pseudocode (Section 4.1).
> > You will see that the ingress state of the packet is (Segments Left ==
1 and Destination Address == the PSP node's address).
>
> Exactly my point. With SL == 1, you are not at the ultimate destination,
so according to what I'll call "Fernando's reading" of RFC8200, you are not
entitled to delete the header. That is the point that IMHO needs to be stated
explicitly in the draft. You are using "Darren's reading" of RFC8200.
>
> I really think you need to say so explicitly. Something like:
>
> Note: this behavior does not contravene section 4 of [RFC8200]
> because the current destination address of the incoming packet
> is the address of the node executing the PSP behavior.
>
> This will not change the argument, but it will make the issue clear so
that we (the IETF) can decide whether to accept it or not. And that is
orthogonal to whether RFC 8200 is wrong.
3)
Given that there is even heated discussions on the text/wording in RFC8200,
I'm afraid that whatever text be inserted in
draft-ietf-spring-srv6-network-programming would trigger controversy and that
in the end be asked to be removed.
However, I'm also in favor of making the issue clear.
Brian, I would personally prefer to stick as close as possible to the text
from RFC 8200, plus that we make clear that we are not changing it but only
referring to. So I would propose something along the following:
As indicated in RFC 8200 section 4, extension headers [...] are
not [to be]
processed, inserted, or deleted by any node along a packet's
delivery
path, until the packet reaches the node [...] identified in
the Destination Address field
of the IPv6 header.
The PSP behavior defined in this section specifies that, the
node identified in the Destination Address field of the IPv6 header (as
restricted by RFC 8200 section 4), when Segment Left is received as 1 and when
specifically instructed by the source node, removes the SRH header.
As a reminder, the text from RFC 8200 is
" Extension headers (except for the Hop-by-Hop Options header) are not
processed, inserted, or deleted by any node along a packet's delivery
path, until the packet reaches the node (or each of the set of nodes,
in the case of multicast) identified in the Destination Address field
of the IPv6 header."
I believe that my proposed edits are clearly indicated by "[]" and helps
the reader to focus on the relevant text. However, I'm also fine with copy
pasting the text verbatim from RFC 8200.
4) There is an interesting comment on the 6MAN WG
https://mailarchive.ietf.org/arch/msg/ipv6/AJzOX97mUeHjEcDSIgpqUw8gNk0/
Although not sent in the SPRING WG, and not mentioning to this document, I
think that I should be taken into account.
More specifically
" IMHO, any specification breaking AH (e.g., by modifying the NextHeader in
transport mode) should clearly note that it 'breaks AH' or constraints its use;
but, this is still acceptable for an IETF standard specification IMHO to 'break
AH'."
I'd propose:
OLD:
As a reminder, [I-D.ietf-6man-segment-routing-header] defines in
section 5 the SR Deployment Model within the SR Domain [RFC8402].
Within this framework, the Authentication Header (AH) is not used to
secure the SRH as described in Section 7.5 of
[I-D.ietf-6man-segment-routing-header].
NEW:
Removing the SRH requires modifying the Next Header field which is defined
as Immutable by RFC 4302. As indicated in
[I-D.ietf-6man-segment-routing-header], the use of RH with AH by an SR source
node, and processing at a SR
segment endpoint node is not defined in this document. Future documents
may define use of SRH with AH and its processing. Such future document needs to
take into account that the use of PSP requires the Next Header field to not be
Immutable.
PC2: Given the following text from RFC4301, and the quote above from the SRH,
do we really need this?
IPsec implementations MUST support ESP and MAY
support AH. (Support for AH has been downgraded to MAY because
experience has shown that there are very few contexts in which ESP
cannot provide the requisite security services. Note that ESP can be
used to provide only integrity, without confidentiality, making it
comparable to AH in most contexts.)
Or whatever text indicating the issue.
Regards,
--Bruno
>
> Regards
> Brian
> >
> > Many thanks,
> > Pablo.
> >
> > -----Original Message-----
> > From: ipv6 <ipv6-boun...@ietf.org> on behalf of Brian E Carpenter
<brian.e.carpen...@gmail.com>
> > Date: Monday, 2 March 2020 at 20:34
> > To: "Darren Dukes (ddukes)" <ddukes=40cisco....@dmarc.ietf.org>, 6man
WG <i...@ietf.org>, SPRING WG List <spring@ietf.org>
> > Subject: Re: PSP and a logical application of RFC8200
> >
> > Darren,
> >
> > Regardless of whether you accept Fernando's comment about the
intention of RFC 8200, there is also the fact that the description of the PSP
flavor cheats by considering the packet to have
> > (Segments Left == 0 and Destination Address == the PSP node's
address).
> > In fact that is *never* the state of the packet on the wire, which
is either
> > (Segments Left == 1 and Destination Address == the PSP node's
address)
> > or
> > (Segments Left == 0 and Destination Address == the final node's
address)
> >
> > OK, maybe it's not cheating, maybe it's only a side effect of the
pseudocode, but the fact is that the test "S14.1. If (Segments Left == 0) {"
in section 4.16.1 is very confusing because it's applied to a packet that is
half way through processing of the routing header (Segments Left has been
updated, but Destination Address has not been updated). This makes it very
unclear how the spec is claiming to interpret RFC 8200.
> >
> > Regards
> > Brian Carpenter
> >
> > On 03-Mar-20 03:52, Darren Dukes (ddukes) wrote:
> > > What follows has been made clear on the list for a while,
> > > I am re-stating it.
> > >
> > > The draft-ietf-spring-srv6-network-programming PSP behavior
> > > strictly follows the letter of RFC 8200.
> > >
> > > RFC8200 section 4 says:
> > >
> > > Extension headers (except for the Hop-by-Hop Options header)
are not
> > > *processed, inserted, or deleted* by any node along a
packet's delivery
> > > path, until the packet reaches *the node* (or each of the
set of nodes,
> > > in the case of multicast) *identified in the Destination
Address field*
> > > * of the IPv6 header.*
> > >
> > >
> > > The processing, insertion and deletion restrictions only apply
> > > “until the packet reaches the node identified in the Destination
> > > Address field of the IPv6 header”.
> > >
> > > At the penuptimate segment of the segment list, the endpoint IS
> > > “the node identified in the Destination Address field of the IPv6
> > > header” and hence the PSP operation programmed by the source SR
> > > node strictly follows the letter of RFC 8200.
> > >
> > >
> > > Thanks,
> > > Darren
> > >
> > >
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